The present invention generally relates to bandstop filters.
Frequency tunable bandstop filters are widely used to remove undesirable signals. Most bandstop filters are reflection type filters, meaning that at the frequency where the filter rejects a signal, that rejected signal is reflected back to the input. This is undesirable for many reasons. For example, consider a microwave or radio frequency receiver that receives a signal through a RF port.
If the receiver utilizes a frequency mixer it most likely uses a bandstop filter to reject unwanted signals. When the rejected signal is reflected by the bandstop filter, it may leak into the RF port. Once the rejected signal leaks back to the RF port, it may be remixed into the input signal, which is known as self-mixing and is very undesirable across most applications.
Recently, reflection-less or absorptive bandstop filters have been proposed which overcome the basic limitation of reflection type filters. Through the design of lossy resonators, absorptive bandstop filters can achieve a theoretically infinite amount of attenuation.
The current state of the art absorptive filters are acceptable for low frequency applications, but for high frequency applications, realization is difficult due to the parasitic and self-resonance effects from the large number of hardware elements required. For example, U.S. Pat. No. 7,323,955 to Jachowski and U.S. Pat. No. 8,981,873 to Zhang et al., each disclose a narrow-band absorptive bandstop filter that uses a plurality of inductors to generate a phase shift for destructive interference. The main drawback of these designs, however, is that as the quality factor and operating frequency changes, the values of the resonators change with exception to the lossy resistors. In other words, the only way to increase the Q is to increase the values of the inductors used. This can be prohibitive at higher RF frequencies.
Accordingly, for at least the foregoing reasons there exists a need for an absorptive bandstop filter that can have an improved Q without prohibitively large inductors at higher RF frequencies.